1. Field of the Invention
The present invention relates to a voltage output device for outputting an analog voltage, for example for use as a reference voltage, of which the value is adjustable.
2. Description of the Prior Art
FIG. 3 shows a conventional voltage output device that outputs a voltage of which the value is adjustable. In FIG. 3, reference symbols R-0, R-1, R-2, . . . , R-i, R-(i+1), . . . , R-(nxe2x88x921), R-n, and R-(n+1) represent resistor elements that have a high resistance, and reference symbols H-1, H-2, . . . , H-i, H-(i+1), . . . , H-(nxe2x88x921), and H-n represent fuse elements that have a sufficiently low resistance relative to the resistance of the resistor elements R-0 to R-(n+1) and that blow when a current higher than a predetermined value is passed therethrough for a duration longer than a predetermined time period.
The resistor elements R-0, R-1, R-2, . . . , R-i, R-(i+1), . . . , R-(nxe2x88x921), R-n, and R-(n+1) are connected in series between a supply voltage VDD and ground GND. Here, the resistor elements R-0, R-1, R-2, . . . , R-i, R-(i+1), . . . , R-(nxe2x88x921), R-n, and R-(n+1) are arranged in the order named from the supply voltage VDD side. The node between the resistor elements R-i and R-(i+1) is connected to an output pad P-OUT.
The fuse elements H-1, H-2, . . . , H-i, H-(i+1), . . . , H-(nxe2x88x921), and H-n are connected in parallel with the resistor elements R-1, R-2, . . . , R-i, R-(i+1), . . . , R(nxe2x88x921), and R-n, respectively.
The supply voltage VDD side ends of the individual fuse elements H-1, H-2, . . . , H-i, H-(i+1), . . . , H-(nxe2x88x921), and H-n are connected to pads P-1, P-2, P-3, . . . , P-i, P-(i+1), . . . , P-(nxe2x88x921), and P-n, respectively, and the ground voltage GND side end of the fuse element H-n is connected to a pad P-(n+1).
In this circuit configuration, at the output pad P-OUT a voltage is obtained as a result of the supply voltage VDD being divided by the resistor elements R-0 to R-(n+1). As long as the fuse element H-k (where k=1, 2, . . . , n) is not blown, since the fuse element H-k has a lowe resistance, the resistor element R-k remains in a short-circuited state. By contrast, when the fuse element H-k is blown, the resistor element R-k is released from the short-circuited state, and thus a change occurs in the voltage division ratio in which the supply voltage VDD is divided.
When a predetermined voltage is applied between the pads P-k and P-(k+1), a predetermined current flows through the fuse element H-k, and thus the fuse element H-k blows. Accordingly, by applying the predetermined voltage between two appropriate pads, it is possible to adjust the output voltage to a desired value.
However, in this conventional voltage output device, it is necessary to provide as many pads for blowing fuse elements as the number of fuse elements plus one, and this requires that the chip area be made larger in proportion to the number of fuse elements. Moreover, to permit adjustment of the output voltage after packaging, the individual pads for blowing fuse elements need to connected to pins. Thus, it is necessary to additionally provide as many pins as the number of fuse elements plus one, and this requires that the number of pins be increased in proportion to the number of fuse elements.
An object of the present invention is to provide a voltage output device that requires minimal chip area and a minimal number of pins.
To achieve the above object, according to the present invention, a voltage output device is provided. The device outputs an analog voltage obtained by dividing a supply voltage with a plurality of resistor elements. The device includes fuse elements, which are blowable when a current higher than a predetermined value is passed therethrough and having resistance sufficiently lower than the resistor elements. The fuse elements are connected in parallel with the resistor elements. The device is formed as an integrated circuit device that permits particular ones of those fuse elements to be blown to adjust the output analog voltage. The device is further provided with switching elements connected in parallel with the fuse elements and control means that controls, in accordance with a signal fed thereto, the on/off states of the switching elements.
This circuit configuration permits the fuse elements to be blown in accordance with a signal fed in from outside, and thus requires only a fixed number of pads for blowing fuse elements irrespective of the number of fuse elements.